Perpendicular Spin-Transfer Torque Memory (STTM) devices operate by driving an electrical current through a pair of ferromagnetic electrodes separated by an oxide-based spacer. The current applies a torque, a spin-transfer torque, on the magnetic moment of electrodes in a memory cell to put a free layer in a parallel or an anti-parallel state with respect to a pinned layer. The direction of the torque depends on the polarity of the current. The amount of torque is related to the amount of current that is driven through the electrodes and the amount of time that the current is applied. For high speed memory which is desired for computer and other data processing applications, a high current is desired in order to reduce the required amount of time. However, higher currents increase power consumption.
STTMs have conflicting design demands. On the one hand, a strong magnet and a large volume make the state of the magnetic memory element more stable but harder to switch. On the other hand, a weak magnet and a small volume make the cell easily switchable by passing a smaller current through the pinned layer but make the device less stable. Accordingly, the strength of the magnet is balanced by its size. A variety of magnet designs materials and compositions have been developed in an effort to find an adequate balance between these two properties.